That is possible, which is capable of being true, since external circumstances are no hindrance to its being true; as for instance, “Diocles lives.” And that is impossible which is not capable of being true; as for instance, “The earth flies.” That is necessary which, being true, is not capable of being false; or perhaps is intrinsically capable of being false, but still has external circumstances which hinder its being false, as for instance, “Virtue profits a man.” That again, is not necessary, which is true, but which has a capacity of being false, though external circumstances offer no hindrance to either alternative; as for instance, “Dion walks.”
That is a reasonable or probable proposition, which has a great preponderance of opportunities in favour of its being true; as for instance, “I shall be alive to-morrow.” And there are other different kinds of propositions and conversions of them, from true to false, and re-conversions again; concerning which we must speak at some length.
XLIX. An argument, as Crinis says, is that which is composed of a lemma or major premiss, an assumption or minor premiss, and a conclusion; as for instance this, “If it is day, it is light;” “But it is day, therefore it is light.” For the lemma, or major premiss, is, “If it is day, it is light.” The assumption, or minor premiss, is, “It is day.” The conclusion follows, “Therefore it is light.” The mode of a proposition is, as it were, a figure of an argument, as for instance, such as this, “If it is the first, it is the second; but it is the first, therefore it is the second.”
A conditional syllogism is that which is composed of both the preceding arguments; as for instance, “If Plato is alive, Plato breathes; but the first fact is so, therefore so is the second.” And this conditional syllogism has been introduced for the sake, in long and complex sentences, of not being forced to repeat the